[Scott Moody]

So I shot some before and after video of my HEI ignition timing during a dyno pull. Surprisingly it jumps around a lot, and also reversed itself at higher rpms. It improves after installing an MSD weight kit, but still not perfect. Dustributor endplay is around .018. Roller cam endplay is around. 010. GM HEI with DUI rev limiting module and external coil.

Edit: used distributor, taken apart and cleaned, shaft bushings "looked" ok

Before:

After:

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[Jayhawk500]

I watched a video of a distributor on a Sun distributor tester. The higher in RPM's the distributor went the more erratic the timing got.
It ended up being the shaft bushings were too loose. The shaft was wobbling around in the hole at the higher RPM's.
You didn't say if this was a new or used distributor. Corvette Central sells all of the components needed to rebuild the dizzy.

This isn't the actual video I watched, but listen to this guys comment at about 2:42.
Sun 404 distributor machine testing a GM HEI distributor - YouTube

 

[David Ray]

First off, ALL large HEI curve kits are NOT FOR STREET USE, THEY ARE FOR DRAG RACE USE ONLY. Those curve kits are designed to disable the vacuum advance and go initial and mechanical curve for total ONLY.They also do not like lighter spring sets that will bring the curve back down to stop at idle speeds, they like to idle far too high for street use.

Here is a partial listing on mechanical curve full degrees offered by OEM GM large HEI weight/center combo's. This is only a partial list, there are just at 437 different combo's available, not all are listed below..ALL, save for the ZZ, are EMISSIONS numbers, what goes on between numbrs given makes it work.

Centerplate# ========== Weight# ===== Crankshaft Degrees*
...#346================139==============24*=====
...#349================139==============20*=====
...#361================139==============24*=====
...#363================139==============15*=====
...#364================139==============18*=====
...#365================139==============06*=====
...#368================139==============20*=====
...#369================139==============22*=====
...#370================139==============25*=====

...375=================41===============22*===== ZZ crate engine combo

...#375================139==============26*=====
...#383================105==============24*=====
...#389================139==============21*=====
...#393================139==============07*=====
...#398================139==============24*=====
...#399================139==============25*=====
...#400================139==============14*=====
...#401================139==============24*=====
...#402================139==============26*=====
...#404================139==============16*=====
...#406================139==============21*=====
...#416================139==============24*=====
...#417================139==============24*=====
...#422================139==============20*=====
...#432================139==============24*=====
...#438================139==============18*=====
...#444================139==============21*=====
...#445================139==============19*=====
...#446================139==============17*=====
...#449================139==============24*=====
...#451================139==============20*=====
...#452================139==============24*=====
...#459================139==============25*=====
...#462================139==============19*=====
...#467================139==============24*=====
...#469================139==============21*=====
...#470================139==============24*=====
...#481================139==============14*=====
...#482================139==============24*=====
...#483================139==============24*=====
...#485================139==============25*=====
...#486================139==============22*=====
...#508================139==============24*=====

 

[Droptop72]

I don't know what RPM you getting to on your dyno pulls, but back in the day when I had access to a distributor machine, I tested a BUNCH of HEI units, and almost all of them fell off rather badly above 5,000 RPM. Usually the dwell would go all out of spec. One that was a little sloppy would lay down even sooner.

 

[nashville beth]

Here's the ticket : https://shop.advanceautoparts.com/p...gXE1Mvfg1HOpcIr0UFUaAvr0EALw_wcB&gclsrc=aw.ds
It's the GM performance HEI used in ZZ502 and other roller cammed crate engines : #93440806 HEI
Use the 20% off code at the top; final price around $125. That's a great deal

 

[David Ray]

"I don't know what RPM you getting to on your dyno pulls, but back in the day when I had access to a distributor machine, I tested a BUNCH of HEI units, and almost all of them fell off rather badly above 5,000 RPM. Usually the dwell would go all out of spec. One that was a little sloppy would lay down even sooner"

Well what does everyone expect? The large coil in cap HEI was designed to do a few things other systems wouldn't, so far. Emissions settings that did not change for at least 50K miles, with little to NO maintenance, decent spark output for a .045 maximum spark plug gap, and, most important, grandma's station wagon grocery getter daily driver, that never saw over 3k in revs, EVER.

THAT is what we built, NOT A TOP FUEL, FUNNY CAR BILLION VOLT SPARKER. It also cannot be modified much past what we built, to make it into a TOP FUEL, FUNNY CAR IGNITION, EITHER.

Live with it.

 

[Georgia69]

Erratic timing can be too much end play on the distributor shaft, can be the cam moving forward and back, can be timing chain slop, etc. Lots of things can cause it.

 

[70GS455]

I don't know what RPM you getting to on your dyno pulls, but back in the day when I had access to a distributor machine, I tested a BUNCH of HEI units, and almost all of them fell off rather badly above 5,000 RPM. Usually the dwell would go all out of spec. One that was a little sloppy would lay down even sooner.

Pulls were to 6000. Surprisingly enough, most of the bounce would occur at a mid rpm range, and improved as it got near 6k.

I replaced both bushings (what a chore, will send that out next time, but it needed it, bottom one was sloppy), reduced endplay to about .012", replaced the worn gear with a hardened steel (apparently the 1050 cam gear material was not compatible with the OEM gear). All of that improved things quite a bit. I think cam endplay is the culprit, I will have a chance to address that as it has to come apart anyway due to a spun rod bearing.

Here are the screw stops that limits mech travel to about 10.5 deg (21 mech degree at the crank)

 

[Droptop72]

"I don't know what RPM you getting to on your dyno pulls, but back in the day when I had access to a distributor machine, I tested a BUNCH of HEI units, and almost all of them fell off rather badly above 5,000 RPM. Usually the dwell would go all out of spec. One that was a little sloppy would lay down even sooner"

Well what does everyone expect? The large coil in cap HEI was designed to do a few things other systems wouldn't, so far. Emissions settings that did not change for at least 50K miles, with little to NO maintenance, decent spark output for a .045 maximum spark plug gap, and, most important, grandma's station wagon grocery getter daily driver, that never saw over 3k in revs, EVER.

THAT is what we built, NOT A TOP FUEL, FUNNY CAR BILLION VOLT SPARKER. It also cannot be modified much past what we built, to make it into a TOP FUEL, FUNNY CAR IGNITION, EITHER.

Live with it.

Just to be clear, I wasn't bitching, I was just stating a fact, and cautioning people from having too high of expectations of what is as you stated, basically a "grocery getter' distributor. HEI distributors are perfectly fine for many applications, including mine, but you will likely either need to do some work or go another route if you need performance above that threshold. It WOULD have been nice if a high performance version would have been developed at the factory, along the lines of the old cast iron tach drive Corvette distributors of the 60's. I remember testing a couple of those that were solid as a rock at the very limit of the distributor machine.

 

[427L88]

I do recall them pretty much being "poochy" when we stabbed them into our old skool solid cammed 7000 rpm engines in the 70's. My buddy went Unilite with the 427, I had a local shop recurve the HEI, MSD triggered it, and ran a Mallory coil off the firewall for the 350 .That let the LT1 see 7000 without rolling over or seeming like the valves were floating.

 

[70GS455]

Just to be clear, I wasn't bitching, I was just stating a fact, and cautioning people from having too high of expectations of what is as you stated, basically a "grocery getter' distributor. HEI distributors are perfectly fine for many applications, including mine, but you will likely either need to do some work or go another route if you need performance above that threshold. It WOULD have been nice if a high performance version would have been developed at the factory, along the lines of the old cast iron tach drive Corvette distributors of the 60's. I remember testing a couple of those that were solid as a rock at the very limit of the distributor machine.

I don't think it was taken as bitching, at least not by me. But you bring up a good point. Testing has shown me that you shouldn't expect one in passenger car configuration to perform flawlessly in a high performance situation, but with some careful thought and the usual changes/improvements it can be a solid performer.

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[pannetron]

I've heard that the HEI distributors are very sensitive to the clearance between the star wheel and the magnetic pickup. Too much clearance leads to erratic timing.

 

[David Ray]

That module..........................should be.deleted, right into the trash collection device, stocker works to 4k just fine, coil in cap is junk.

YES, reluctor gap is extremely sensitive, and when the pickup coil goes away, replace the entire pickup assembly, NOT JUST THE COIL. It takes an alignment tool to get the outer reluctor re-centered so it doesn't hit the inner reluctor.

Aftermarket curve kits are not for street use, drag race ONLY. To get the right mechanical curve, select one of the 437 different curve sets, weights/center package. Adding screws,welding, brazing onto the rotor mount is simply "I haven't got a clue" stuff, get the right weights/center packet, and do it the right way in the first place.

These large coil in cap HEI's were never designed, nor intended to do anyhting more than go 50K with low, to no maintenance, and no timing changes along the way, they are EMISSIONS distributors, NOT performance ones. The ONLY performance large HEI's are from the ZZ crate engine series, with the GM OEM "990 Series" modules.

 

[hydro462]

I like the Stock Iron distributor with a manual tach connector just being used as a CD box trigger is a fantastic ignition..